1. Field of the Invention
The present invention relates to a lithium secondary battery, for example a lithium secondary battery comprising a negative electrode capable of intercalating/deintercalating lithium ion, a positive electrode made of a lithium-containing metal oxide as an active positive material, a nonaqueous electrolyte and a separator for separating the positive electrode and the negative electrode from each other and a battery device comprising same.
2. Description of the Related Art
Recently, electronic apparatus have shown a remarkable reduction of size and weight thereof. Under these circumstances, it has been keenly desired to reduce the size and weight of battery as power supply. Accordingly, a lithium secondary battery such as lithium ion battery has been put to practical use as rechargeable battery having a small weight and high capacity. Such a lithium secondary battery has been used for portable electronic and communications apparatus such as small-sized video camera, portable telephone and note type personal computer.
This type of a lithium secondary battery comprises as an active negative electrode material a carbon-based material capable of intercalating/deintercalating lithium ion, as an active positive electrode material a lithium-containing metal oxide such as LiCoO2, LiNiO2, LiMn2O4 and LiFeO2, and an electrolytic solution obtained by dissolving a lithium salt as a solute in an organic solvent. These components are assembled into a battery. When this battery is charged for the first time, lithium ions eluted from the active positive electrode material enter in the carbon particles to make the battery chargeable and dischargeable.
As such a lithium secondary battery is overcharged, excess lithium ions are extracted from the positive electrode while excess lithium ions are intercalated in the negative electrode, causing the precipitation of metallic lithium. Extremely unstable high oxides are produced on the positive electrode side, where lithium ions have been lost. Further, when the battery voltage reaches higher than about 5.0 V during overcharging, the battery undergoes decomposition reaction of the organic solvent of the electrolytic solution that causes the production of a large amount of combustible gas and sudden exothermic reaction that causes abnormal heating of the battery, impairing the battery safety. These circumstances cause an important problem as the energy density of the lithium secondary battery increases.
As mentioned above, when a phenomenon involving abnormal heating of battery occurs, the constituent materials of battery such as positive electrode, negative electrode and electrolytic solution undergo denaturation that makes it impossible to maintain the desired battery properties. Further, this phenomenon should be avoided from the standpoint of maintenance of battery safety. Accordingly, at present, this type of secondary lithium batteries are always used with a protective circuit for preventing overcharging incorporated therein.
A nickel-cadmium storage battery or the like is arranged to prevent overcharging through subtle utilization of a gas absorption mechanism that oxygen produced at the positive electrode side during overcharging is reacted with hydrogen at the negative electrode side to produce water. However, a lithium secondary battery comprising an organic solvent is almost unable to utilize such a gas absorption mechanism in principle at present. It is thus inevitably necessary that a lithium secondary battery comprising an organic solvent comprise a protective circuit for preventing overcharging incorporated therein.
However, the protective circuit for preventing overcharging requires a complicated control technique and thus becomes a cause that adds to the total cost of the battery. Further, since practically used secondary lithium batteries have a protective circuit device provided in a battery pack, the occupying volume and weight of the protective circuit also becomes a cause that reduces the substantial energy density, particularly volume energy density (Wh/m3), of the battery.
Under these circumstances, the inventors made studies. As a result, it was found that if a lithium secondary battery which cannot be overcharged is obtained, it is not necessary to provide a protective circuit for preventing overcharging. The present invention has been worked out on the basis of this knowledge. It is therefore an object of the invention to provide a lithium secondary battery which cannot be overcharged and a battery device which comprises such a lithium secondary battery to eliminate the necessity of providing a protective circuit therein.
In order to accomplish the foregoing object of the invention, the lithium secondary battery of the invention comprises a negative electrode capable of intercalating/deintercalating lithium ion, a positive electrode made of a lithium-containing metal oxide as an active positive material, and a nonaqueous electrolyte, wherein a polyvinylidene fluoride resin is disposed between said negative electrode and positive electrode so that the battery voltage doesn""t rise beyond a predetermined value even when said battery is overcharged.
Preferably the lithium secondary battery of the invention comprises a negative electrode capable of intercalating/deintercalating lithium ion, a positive electrode made of a lithium-containing metal oxide as an active positive material, a nonaqueous electrolyte and a separator comprising a polyvinylidene fluoride resin for separating the positive electrode and the negative electrode from each other, whereby the battery voltage doesn""t rise beyond a predetermined value, i.e., 5.0 V, even when the lithium secondary battery is overcharged.
A lithium secondary battery was prepared by disposing a negative electrode capable of intercalating/deintercalating lithium ion (e.g., graphite) and a lithium-containing metal oxide (e.g., lithium-containing cobalt oxide) with a separator comprising a polyvinylidene fluoride (PVdF) provided interposed therebetween, and then injecting a non-aqueous electrolyte thereinto. When the inventors made an overcharging test (overcharging with electricity corresponding to at least 400% of the battery capacity) on this lithium secondary battery, a surprising phenomenon was shown that the battery voltage doesn""t rise beyond a predetermined value (particularly 5.0 V) (see FIG. 5).
The reason for this phenomenon is not made obvious at present. However, it can be presumed as follows. In other words, a polyvinylidene fluoride (PVdF) resin has excellent electrical insulation properties and is highly capable of retaining an electrolytic solution thereinside to exhibit a high ionic conductivity. Thus, a polyvinylidene fluoride (PVdF) resin provides an excellent separator material. A battery prepared from such a polyvinylidene fluoride (PVdF) resin as a separator material shows a change of the chemical or electrical properties of the polyvinylidene fluoride (PVdF) resin when the battery voltage shows an abnormal rise to about 4.5 V upon overcharging. In this manner, current flowing upon overcharging is consumed through a reaction process different from the inherent charging reaction of active positive electrode material or active negative electrode material.
The mechanism of the establishment of such a separate reaction process can be considered as follows, though being inference:
i) When the battery is overcharged, the polyvinylidene fluoride (PVdF) partly reacts with a part of lithium or electrolytic solution to form a reaction seed taking part in a reversal electrochemical reaction (redox reaction). The reaction seed migrates between the positive electrode and the negative electrode to inhibit other reactions (charge reaction).
ii) The same participation as in the mechanism (i) causes decomposition reaction of solvent accompanied by no production of gas.
iii) When the battery is overcharged, the electrical insulation of the polyvinylidene fluoride (PVdF) is destroyed to form an electronic conductor having a certain resistivity.
Thus, there occurs an internal short-circuiting only when the battery is overcharged. Accordingly, an ordinary electrochemical reaction (reaction of electron with ion) doesn""t occur. Therefore, the polyvinylidene fluoride (PVdF) acts as a mere resistor.
From the standpoint of the foregoing mechanism, when a battery device is formed by a lithium secondary battery which shows no rise of battery voltage beyond a predetermined value even when overcharge, the same overcharging properties as obtained with a protective circuit for preventing overcharging can be obtained without being provided with any such a protective circuit. This eliminates the necessity of packaging the battery with such a protective circuit as practiced conventionally. Thus, the volume of the protective circuit required to occupy in the container or battery packaging container becomes unnecessary. Accordingly, the substantial energy density of the battery (particularly volume energy density: Wh/m3) can be enhanced.
By thus making the protective circuit unnecessary, this type of battery devices can be produced through less production steps and hence at a reduced cost.